1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point through a wire, a wire bonding apparatus for carrying out the method, a wire loop having a certain shape and a semiconductor device having such a wire loop incorporated therein.
2. Description of the Related Art
Conventionally, in a process of fabricating a semiconductor device, as shown in FIG. 7A or 7B, wire bonding for connecting a pad 2a or a first bonding point A of a semiconductor chip 2 attached to a lead frame 1 and a lead 1a or a second bonding point Z of the lead frame 1 through a wire 3 has been carried out. Typically, loop shapes of the wire 3 connecting the first and second bonding points A and Z include a trapezoidal shape and a triangular shape shown in FIGS. 7A and 7B, respectively, as disclosed, for example, in U.S. Pat. No. 6,036,080 or Japanese Patent Application Laid-Open Publication No. 2000-277558.
The wire loop having a trapezoidal shape shown in 7A is formed by a sequence of steps as shown in FIG. 8. First, in step (a) of FIG. 8, a capillary 4 through which the wire 3 passes is lowered and a ball 30 which has been formed at a tip end of the wire 3 is bonded to the pad 2a of the chip 2 or first bonding point A. Then, in step (b) of FIG. 8, the capillary 4 is vertically raised to a point B while the wire 3 is delivered. Thereafter, in step (c) of FIG. 8, the capillary 4 is horizontally moved to a point C in a direction opposite from the second bonding point Z.
In general, such an operation of the capillary 4 to be moved in the direction opposite from the second bonding point Z is referred to as a “reverse operation”. As a result, the portion of the wire 3 between the points A and C is formed to be inclined and the wire 3 is formed at an upper end of the inclined portion thereof with a kink 3a by a lower end of the capillary 4. The portion of the wire 3 between the points A and C thus delivered corresponds to the height of a neck portion H (or a portion of the wire 3 between the pad 2a and the kink 3a) and will constitute the neck portion H.
Subsequently, in step (d) of FIG. 8, the capillary 4 is vertically raised to a point D while the wire 3 is delivered. Then, in step (e) of FIG. 8, the reverse operation of the capillary 4 is performed again, i.e. the capillary 4 is horizontally moved to a point E in the direction opposite from the second bonding point Z. As the result of this reverse operation, the wire 3 has another inclined portion extending between the points C and E, and a kink 3b is formed in an upper end of this inclined portion of the wire 3.
This inclined portion of the wire 3 thus delivered will constitute an upper base portion L (or a portion of the wire 3 between the kinks 3a and 3b) of the wire loop having a trapezoidal shape shown in FIG. 7A. Thereafter, in step (f) of FIG. 8, the capillary 4 is vertically raised to a point F so that the wire 3 is delivered by a length corresponding to a long inclined portion S (or a portion of the wire 3 between the kink 3b and the lead 1a) of the wire loop shown in FIG. 7A. Subsequently, the capillary 4 is lowered to the second bonding point Z via positions f1 and f2, so that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
The wire loop having a triangular shape shown in 7B is formed by a sequence of steps as shown in FIG. 9. Since the wire loop having a triangular shape is not provided with an upper base portion (L) unlike the wire loop having a trapezoidal shape described above, in forming the wire loop of a triangular shape, the second reverse operation in steps (d) and (e) of FIG. 8 is not conducted. Therefore, in this instance, a step that corresponds to the steps (d), (e) and (f except for f1 and f2) of FIG. 8 is carried out only in step (d) of FIG. 9. More particularly, steps (a), (b) and (c) of FIG. 9 are the same as the steps (a), (b) and (c) of FIG. 8, and after the first reverse operation in step (c) of FIG. 9, the capillary 4 is vertically raised to a point F in step (d) of FIG. 9 while the wire 3 is delivered. Subsequently, in step (e) of FIG. 9, the capillary 4 is moved via positions e1 and e2 in a manner similar to that in step (f) of FIG. 8, with the result that the wire 3 is bonded to the second bonding point Z or the lead 1a. 
However, in the above-described techniques, as the wire loop includes the neck portion H having a somewhat large height, the wire loop becomes high and thus is rendered to be unstable. In addition, in a case where a wire loop is formed without any reverse operation of the capillary in order to make the height of a neck portion H thereof small and the height of the neck portion H is reduced to a certain level or below, the neck portion H is liable to be damaged in drawing or moving the wire 3 to arrange it in place because of the wire 3 vertically extending from the first bonding point A.